Prop Lynch has enjoyed an outstanding season in the Hull reserve side and was close to earning a full time contract under Richard Agar, but has decided to make the move to the Chris Moyles Stadium.
Welham, younger brother of established Rovers centre Kris, also joins up with Daryl Powell’s squad.
A statement on the club’s website read: “Tommy is a quality young player who was close to landing a full time contract and has a real desire to prove himself.
“Liam is a young centre with pace, who hits great lines in attack and is a strong defender when needed. Featherstone’s coach, Daryl Powell, sees both players as capable of adding a massive amount of depth and quality to the squad.”